Although all
Visa cards start with the number 4, the following 5 digits indicate which bank
that issued the Visa card. Mastercards start with 5, Discover starts with 6.
You’ll notice that some websites don’t ask you what type of card you have —
they obviously know what the digits on the card mean.

Digits 7to 15
(Account ID)

The unique number that identifies your account

Digit 16
(Checksum)

This single
digit is one of the most important ones on the card. Much like the last digit of a barcode,
the sole purpose of this digit is to allow validation of the rest of the
number. In other words, there is a mathematical relationship between the
numbers on the card, so if the number is entered incorrectly, the card
validator system can indicate the card number was entered incorrectly.